1. Field of the Invention
The present invention relates to a data communication system for transmitting and receiving motion (moving) image data. More particularly, it relates to a system for coding and transmitting motion image data, such as image data of a television (TV) telephone (conference) system, with a high efficiency in a form of interframe (frame-to-frame) difference coded signals or in a form of motion compensation interframe code signals.
2. Description of the Related Art
Generally, the image on, for example, a TV display unit, consists of a large amount of image data. In order to transfer such image data economically, a variety of data compression technologies have been attempted. In this connection, the means for reducing redundancy (correlation) in time and space of input image signals, reducing information on amplitude components, and thus expressing the input image signals with as as possible as an amount of data small, are generally known as high efficiency coding or redundant compression coding.
The data compression technologies are briefly classified into two categories: a prediction coding method, such as interframe coding, intra-frame coding, differential pulse coded modulation (DPCM), vector quantization coding, etc.; and an orthogonal transform method, such as a Walsh-Hadamard transform, a discrete Fourier transform, etc. In the prediction coding method, the prediction of picture elements (PIXELs) is effected on the basis of the correlation of adjacent PIXELs, and errors in predicted values are coded so that the amount of information to be transferred for each PIXEL can be reduced. In the orthogonal transform method, an orthogonal transform of PIXEL groups is carried out to obtain statistical deviations of the spectrum given by the transform, and quantization level numbers are suitably assigned to the statistical deviations so that the amount of information to be transferred can be reduced.
In a TV telephone system, image data is changed with a time lapse, but the change of the image data may be a partial for a short period and also may be somewhat slow. On the basis of these features of the image data, a further method which obtains intra-frame or interframe difference signals or motion-compensated (MC) interframe difference signals and codes and transmits these difference signals, is available to further reduce the redundancy of the motion image signals in a time axis direction. The difference methods have been attempted for a system coding PIXELs, and for another system coding blocks each consisting of a plurality of PIXELs and formed in a matrix form.
JPA No. 55-115780, published on Sept. 9, 1980, discloses a prediction coding system for television signals which codes differences for PIXELs and is provided with a noise filter for decreasing quantization errors due to a rough quantization of prediction errors.
JPA No. 55-158784, published on Dec. 10, 1980, discloses an interframe coding apparatus which codes differences for each block consisting of a plurality of PIXELs and reduces the amount of calculation needed by dividing the detection of motion vectors into multisteps.
JPA No. 57-41069, published on Mar. 6, 1982, discloses an interframe coding system which codes each PIXEL or each block and is provided with a code error detection circuit for reducing transfer data errors.
JPA No. 57-199379, published on Dec. 7, 1982, discloses a vector coding apparatus which codes and decodes motion vectors represented by two-dimensional vectors, with a high efficiency.
U.S. Pat. No. 4,077,053 discloses a television signal encoder utilizing a correlation between frames which corrects coding errors in an interframe coding process by low-bit interframe coding with a limited increase in the volume of coding information. The encoder includes a first quantization circuit, a predictor, and a second quantization circuit for detailed quantifying of interframe coding errors output from the first quantization circuit.
As set forth above, prior art MC interframe difference systems provide improved data compression. On the other hand, these prior art systems process either each PIXEL or each block consisting of a plurality of PIXELs. Note, the motion image data of the TV telephone system may have a variety of changes in time. The PIXEL MC interframe difference system and the block MC interframe difference system respectively have advantages and disadvantage with regard to the motion image data. That is, the prior art MC interframe difference systems still suffer from the disadvantage of poor image quality, such as large distortion and noise.
The above disadvantages will be described in more detail with reference to a specific example.
Also, an improvement of the system for achieving a relatively high compression of data, and an inexpensive transmission charge, etc. is required.